When theory meets experiment: What does it take to accurately predict 1H NMR dipolar relaxation rates in neat liquid...

In this contribution, we compute the 1H nuclear magnetic resonance (NMR) relaxation rate of liquid water at ambient conditions. We are using structural and dynamical information from Coupled Cluster Molecular Dynamics (CCMD) trajectories generated at CCSD(T) electronic structure accuracy while also considering nuclear quantum effects in addition to consulting information from x-ray and neutron scattering experiments. Our analysis is based on a recently presented computational framework for determining the frequency-dependent NMR dipole-dipole relaxation rate of spin 1/2 nuclei from Molecular Dynamics (MD) simulations, which allows for an effective disentanglement of its structural and dynamical contributions and includes a correction for finite-size effects inherent to MD simulations with periodic boundary conditions. A close to perfect agreement with experimental relaxation data is achieved if structural and dynamical information from CCMD trajectories is considered, leading to a re-balancing of the rotational and translational dynamics, which can also be expressed by the product of the self-diffusion coefficient and the reorientational correlation time of the H-H vector D0 × τHH. The simulations show that this balance is significantly altered when nuclear quantum effects are taken into account. Our analysis suggests that the intermolecular and intramolecular contributions to the 1H NMR relaxation rate of liquid water are almost similar in magnitude, unlike what was predicted earlier from fully classical MD simulations.